This invention relates to weld guns and more particularly to a control device for a weld gun.
Weld guns are commonly used in industry to weld panels or the like together. In a typical weld gun installation, a pair of arms are pivotally mounted intermediate their ends on a support member, a weld tip is provided at one end of each arm, and a power device such as an air cylinder is positioned between the other ends of the arms. In operation, after the panels or the like to be welded have been positioned between the spaced welding tips, the power device is actuated to move the weld tips in a closing direction and into contact with opposite faces of the panels, the weld tips are electrically energized to accomplish the resistance welding operation, and the power device is actuated in the opposite sense to move the weld tips apart.
Whereas these prior art weld guns have been generally satisfactory, there are situations in which the weld tips damage the finish of the members being welded. Specifically, whereas the arms of the gun in their open position are typically stabilized by positive stops, as the arms move away from the stops and move toward their closed position, they are subject to unbalancing forces. As a result, one weld tip often engages the members to be welded before the other tip and the engaged tip thereafter absorbs the reaction force generated by the continued opening movement of the power cylinder. As the engaged tip becomes the reaction point for the final closing movement of the other tip, it digs into the surface of the member to be welded and leaves a dimple in the surface, or in severe cases, badly deforms the member and/or dislodges the member from the associated fixture. Further, even if the tips arrive at the desired welding location at the same time, the unbalancing forces inherent in the weld gun act to exert a force on the tips tending to move them, in unison, away from the position at which they closed, further tending to damage the panels. In some situations the damage resulting from these unbalancing forces is tolerable but in many situations, for example when welding cosmetic panels such as exterior automobile panels, this damage is not tolerable and requires that the automobile be pulled off of the assembly line and moved to a special repair area where the damage is repaired in an expensive and time consuming operation.